Magneto-repulsion punching with dynamic damping

ABSTRACT

Dimpling of a workpiece by rebounding of a magnetic repulsion punch mechanism, when operated at high punching rates, is avoided by application of a damping pulse at a point at or subsequent to a mid-point of a return stroke of the punch from the workpiece and preferably within the last 10% of the return stroke. This damping pulse is preferably a fraction of the pulse width and of approximately the same amplitude as a driving pulse applying kinetic energy to the punch for causing punching of the workpiece. At high punching rates the kinetic energy of the return portion of a punching stroke is substantially determined by elastic rebounding of the punch from a guide bushing. The removal of kinetic energy during the return portion of the stroke allows the return of the punch to a position close to a rest position during a very short interval with small overshoot under control of mechanical damping. Since dynamic damping may be substituted for much of the mechanical damping in the punching arrangement, high quality punching may be accomplished at much increased punching rates. A preferred form of monitoring punch position employs RF reflected inductance and may be used to form a time expanded image of punch movement.

This application is a continuation of application Ser. No. 07/995,428,filed Dec. 18, 1992, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the formation of apertures insheet material by punching and, more particularly, to the monitoring andcontrol of punching tools operated at high speed, especially as used inthe fabrication of electronic circuit components.

2. Description of the Prior Art

The manufacture of many articles involves the formation of aperturestherein. In particular, it has been the common practice in theconstruction of electrical and electronic devices to mount components ona perforated, insulating board or other substrate by passing leadsthrough apertures in the board. In such a case, connections are oftenformed by a conductive metal pattern on the board and also by wiringpassed through the board, as in constructions involving the well-knownprinted circuit.

In recent years, electronic circuits and devices of greatly increasedcomplexity have been fabricated with multiple layers. One such type ofdevice is the multi-layer module (MLM) which employs a large number ofinsulative layers to separate numerous conductive patterns. Therespective conductive patterns may be selectively connected at holesformed in the insulative layers, commonly referred to as vias.

A particular form of MLM is the multi-layer ceramic (MLC) device inwhich the insulative layers are initially formed of an uncured ceramic,referred to as a green sheet. Conductive patterns are formed thereon byscreening of a conductive paste onto a surface of the green sheet with astencil or mask. Vias may be filled in the same manner or even in thesame screening operation. After the conductive patterns are formed onthe respective green sheets, the green sheets are assembled into anaccurately registered stack and sintered under pressure to cure theceramic and form the complete interconnection arrangement embeddedtherein.

Of course, in such structures, the vias are extremely small and verynumerous in each of the 20 to 100 or more green sheets which may bepresent in a single device. This is particularly true for so-calleddistribution layers which serve to change the nominal conductive featurespacing in the so-called connection layers to a size and spacing (e.g.pitch) compatible with integrated circuits which are mounted thereon.Therefore, the apertures which form the vias must be accuratelypositioned and must be of very accurate geometry. To achieve thisaccuracy, a punch is generally used to form the vias. A punch is alsodesirable because the material from the punched hole will be of arelatively large particle size in comparison with other methods whichcut or erode material from the aperture. Such material can be a sourceof contamination of the green sheet during further processing and largerparticle size facilitates control thereof.

A punch apparatus must be very large in comparison to the holes formedin order to develop the force necessary to successfully and cleanlypunch through the material (e.g. green sheet). The machining of punchesto form multiple holes in a single stroke is more expensive than singlehole punches. Further, the results obtained from multiple hole punchesare generally inferior to the hole quality obtained with a single holepunch in terms of geometric accuracy and positioning accuracy. However,if a single hole punch is used, the number of holes which must be formedin each green sheet dictates that the punch apparatus be operated a veryhigh speed in order to achieve acceptable throughput duringmanufacturing processes.

Since such punches are mechanical devices, although generally operatedby electrical systems, accuracy may be compromised at higher speedswhere mechanical limitations are encountered due to high accelerationand deceleration forces and the geometry of the punch mechanism. Inparticular, when the velocity of the punch is increased, thereciprocating portion of the punch may strike other portions of thepunch structure at the ends of the punching stroke and elasticallyrebound therefrom. Such rebounding is essentially an uncontrolled motionof the punch and, if severe, may cause unintended contact between thepunch and the workpiece. At the high accuracies required in MLCstructures, such contact will almost invariably cause significant damageto the green sheet as dimples in the green sheet surface which willaffect the manufacturing yield of the devices made.

Since the length of the punch stroke must be limited for use at highpunching rates and accuracies, uncontrolled motion due to rebounding isa substantial limiting factor in regard to the punching ratesobtainable. Compensation or avoidance of such effects are non-trivial.While mechanical damping is somewhat effective in reducing rebounding,in practice, it merely allows increase in speed until deleteriouseffects of the rebounding motion are again encountered. Such mechanicaldamping often does not produce substantial gain in punching rates andthe extent to which it can be applied is limited since mechanicaldamping causes significant punch speed reduction during the returnstroke and thus increases the total time duration of a single punchingoperation.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a dynamicdamping arrangement in order to avoid causing uncontrolled punch motionat high speeds and punch rates.

It is another object of the invention to provide a magneto-repulsionpunch drive arrangement capable of operating at speeds not previouslyattainable.

It is a further object of the invention to provide a punchingarrangement for producing holes for vias which are of high geometricalaccuracy and operable at high speeds and punching rates withoutengendering damage to or defects in a workpiece.

It is yet another object of the invention to provide a technique ofmonitoring punch motion during operation of the punch apparatus in orderto optimize punching rates and also provide mechanical diagnostics forthe punch apparatus.

In order to accomplish these and other objects of the invention, amethod is provided for dynamically damping a magnetic repulsionmechanism having a primary stator coil and a mechanically damped movablemember including a secondary coil coupled therewith, in which a drivingpulse of a predetermined amplitude and pulse width causes initialmovement of said movable member, comprising the steps of causing themovable member to rebound from other structure, and applying a dampingpulse to said primary coil during the rebound of the movable member,whereby kinetic energy is removed from the movable member during therebound.

In accordance with another aspect of the invention, a dynamically dampedmagnetic repulsion mechanism is provided including a mechanically dampedmovable member, an arrangement for applying kinetic energy to themovable member and for causing rebounding motion of the movable member,and an arrangement for removing kinetic energy from said movable memberduring a selected portion of the rebounding motion of said movablemember.

In accordance with a further object of the invention, an arrangement isprovided for monitoring relative motion between a primary coil and asecondary coil in a magneto-repulsion system including

means forming a resonant circuit with said primary stator coil,

means for applying an electrical signal to said resonant circuit, and

means for detecting an envelope of a signal appearing at a node of saidresonant circuit as the quality factor of said resonant circuit ischanged by said relative motion between a primary coil and a secondarycoil.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a cross-sectional view of an exemplary punch arrangement towhich the invention may be advantageously applied,

FIG. 2 is an enlarged view of a portion of FIG. 1,

FIG. 3 is a cross sectional view of a green sheet exhibiting dimplingdue to high speed punch operation without employment of the inventionand corresponding electrical drive waveform and graphical depiction ofpunch motion,

FIG. 4 is a graphical depiction of dynamic damping in accordance withthe invention,

FIG. 5 is a schematic diagram of an electrical drive arrangementincluding dynamic damping in accordance with the present invention

FIG. 6 is a schematic diagram of a preferred circuit arrangement formonitoring punch motion,

FIGS. 7 and 8 are graphs useful for explaining the operation of thecircuit arrangement of FIG. 6, and

FIGS. 9 and 10 are profiles obtained in the same manner as FIG. 8 usingthe circuit arrangement of FIG. 6 which are indicative of differentmodes of punch operation or malfunction.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, there isshown, in cross-section, a punching mechanism 10 to which the inventionmay be advantageously applied. This punching mechanism preferablyincludes a die 11 for supporting the workpiece which has an aperture 15therein matching the diameter of punch tip 14. The punch tip 14 ispreferably made from or coated with silicon carbide. This die 11 and theaperture 15 provides a cutting edge against which the punch tip 14passes and further provides for control of the material 47 removed fromthe punched hole. Such material 47 may be disengaged from the punch tip14 by an air jet applied at 19 within the die 11. The punch mechanismalso preferably includes a stripper 17 for avoiding positional error ofthe punch 14 due to vibration or flexure of the punch body or loosenessin guide (e.g. bushing) 16 through which the punch body 14' isreciprocated. Spring 24 is preferably provided to establish a restposition for the punch and to provide some clearance for relative motion(e.g. the downward stroke and close clearance 26) and to support theweight of the punch body and the annular disk 18. Spring 24 also assistsin withdrawing the punch tip 14 from the punched material. However, athigh punch rates the punch is preferably driven at velocities at whichthe punch body 14' will rebound from the bushing 16 with kinetic energyonly slightly reduced from the downward stroke. In this operationalregime, the forces applied by spring 24 may be neglected.

Drive for the punch body 14' and punch tip 14 is provided at the upperend of punch body 14' in the form of an annulus of conductive material18, preferably copper. This annular disk 18 functions as a single turnwinding of high conductivity when linked by a changing magnetic fieldproduced by drive circuit 20 and stator coil 22, preferably formed as amulti-turn winding coaxial with annular disk 18 and the remainder of thepunch mechanism 10. A magneto-repulsion action is produced when arapidly changing current is applied to stator coil 22. Due to the closeclearance 26, the magnetic field produced by stator coil 22 in responseto an energizing pulse is closely coupled to the conductive annulus ofannular disk 18, inducing a current therein by a transformer-likeaction. This current produces a magnetic field emanating from theannular disk 18 and of opposite polarity to the field emanating fromstator coil 22. These two opposing magnetic fields produce a strongrepelling force, causing the annular disk 18 and the attached punch 14to accelerate away from the coils 22 at a rate of several thousand timesthe acceleration of gravity (several thousand G's).

It is important to recognize that this electro-magnetic repulsionmechanism is fundamentally different from a solenoid arrangement inwhich a force is exerted on a preferably ferromagnetic movable member ina direction tending to reduce the reluctance of the magnetic circuitresulting from energization of the coil. While a solenoid is capable ofdeveloping substantial force when the movable member is brought as closeas possible to the coil and inductance is minimized, it is not aseffective as the magnetic repulsion arrangement described above forproducing large accelerations from narrow energization pulses. This isdue, in large part, to the geometry of the respective mechanical systemssince the attraction force developed by a solenoid decreases rapidlywith separation of the movable member and the coil (e.g. maximum forceis developed only when magnetic reluctance is minimized and no furthermovement in a particular direction is mechanically possible). The forcedeveloped by a solenoid is also limited by magnetic saturation. Incontrast, the magnetic repulsion arrangement described above does notrely on ferromagnetic elements in a magnetic circuit but rather on acurrent induced in the highly conductive single turn secondary winding.The magneto-repulsion effect is the repulsion between two magneticfields due to currents in the primary and secondary windings and iseffective over a moderate separation of the primary and secondarywindings corresponding to the ability to induce a substantial current inthe secondary winding. Thus, over the displacement of the punch body 14'where driving and damping pulses are applied in accordance with theinvention, substantial repelling forces can be generated. Therefore,high accelerations may be developed throughout the stroke andparticularly the upper half thereof.

For short pulses, substantially less than the time constant determinedby the inductances of the stator 22 and the annular disk 18, the kineticenergy imparted by the magneto-repulsion effect will be determined bythe pulse width as well as the voltages applied. Therefore, in practice,a drive pulse of about 50 μs is generally used and the driving voltageis varied with the punching rate and the properties (e.g. thickness,resilience, tensile and shear strength, etc.) of the workpiece.

The close clearance 26 between the stator coil and the annular disk 18also provides a significant degree of pneumatic damping. However, theclose clearance which is required for such a function (e.g. littledamping will be produced when the stator 22 and annular disk 18 arewidely separated during the lower half of the downward punching stoke)limits the amount of allowable overshoot as the punch body 14' returnsto its rest position. This limitation on punch position overshoot causesthe rebounding and uncontrolled motion of the punch, referred to above.

The motion of the punch tip 14 is shown in enlarged form in FIG. 2,corresponding to the portion of FIG. 1 enclosed by circle 21. Picking anarbitrary reference point 30 on punch body 14' and recognizing that,during multiple punching operations, relative motion is provided betweenpunch mechanism 10 and the workpiece (e.g. green sheet 12) in thedirection indicated by arrow 40, reference point 30 will follow a pathas illustrated, including points 32, 34, 36, and 38, collectivelyreferred to as path 42. The downward punching stroke 32 carries thepunch tip 14 from the position indicated at solid line 31 (e.g. the restposition) to the position indicated by dashed line 33 to punch a hole46, bringing the reference point to position 34. The impact of the steelbutton 23 against the top surface of guide bushing 16 as well as theinfluence of spring 24 causes the punch to rebound upward to returntoward the rest position 31, bringing the reference point to 36. Thelatter portion of this motion is damped mechanically by motion of theambient atmosphere as annular disk 18 approaches stator winding 22 andovershoot is low. Nevertheless, at high punching rates, thecompressibility of the ambient atmosphere as well as some degree ofimpact between the annular disk 18 and stator winding 22 causes arebounding of the punch as shown by the position of reference point 30at position 38. For purposes of illustration, it is assumed, as it isoften observed at high punching rates, that this rebound motion issufficient to bring punch tip 14 into a second contact with theworkpiece 12. Due to the relative motion 40 during the upward stroke andrebound, punch tip 14 is no longer registered with the punched hole andcontact between the punch tip and workpiece causes a dimple 44 as shownin further enlarged form in FIG. 3.

In FIG. 3, the energizing pulse 41 causing motion through path 42 isshown superimposed thereon. As described above, this energizing pulse,applied to stator coil 22 causes a large downward stroke causingpunching of hole 46 and ejection of slug 47 from the workpiece 12.However, the rebound stroke, from 36 to 38, causes the punch tip to beagain driven against green sheet 12, forming dimple 44.

It can be readily appreciated from FIG. 3, which corresponds to actualphotomicrographs of holes in green sheets, that the dimple, if filledwith conductive material, causes the via to be much larger at the topthan at the bottom. This is equivalent to misregistration of the greensheets when the MLC device is assembled and, if the conductors or viasare closely spaced, could cause shorting therebetween either on the samelayer or an adjacent layer. It should be also noted in regard to theadjacent layers or lamina, that the possibility of shorting may beincreased by slight and otherwise tolerable misregistration of greensheets when stacked for sintering or by unavoidable differentialshrinkage of the green sheets during drying, curing and sinteringprocesses. Therefore, dimples reduce manufacturing tolerances and/ormanufacturing yields of devices so constructed.

Of somewhat lesser importance but still significant in regard to viaformation and manufacturing yield is the deformation of green sheetmaterial, as indicated at 48 as the dimple is formed. Since the volumeof the dimple is not cut away from the green sheet, the material isdeformed principally into the previously punched hole and thus reducesthe cross-sectional area of the hole 46. This constriction of the holenot only increases the resistance of the via at a relatively localizedposition, maximizing deleterious thermal effects in operation of thedevice so formed, but may impede the extrusion of conductive materialinto the hole, resulting in defective via formation.

Referring now to FIG. 4, the dynamic damping technique in accordancewith the invention will be explained. As indicated above, the problemarises from the velocity of the punch body 14' during the return stroke.Simply put, the inventors have discovered that a second pulse related inwaveform to the initial drive pulse and applied during the return strokecan be used to remove kinetic energy from the punch body to a degreethat the system will be sufficiently damped and the rebound can beeither eliminated or sufficiently reduced that dimpling is avoided. Ithas been empirically determined that a damping pulse 56 of the sameamplitude (for circuit simplicity) and about 25% of the width of thedriving pulse 41 will avoid the dimpling effect if applied at a point inthe last 50% of the return stroke (shown in dashed lines) and preferablyin the last 10% of the return stroke, shown at 56'. This can be readilyunderstood since, if the dynamic damping pulse 56 is applied earlier,but, of course, subsequent to the punching (e.g. between t₁ and t₂),magnetic coupling will be reduced. Also, the system speed will bedrastically reduced since the duration of the return stroke will beincreased by reduced punch speed over the remainder of the returnstroke. If the dynamic damping pulse 56 is applied later (e.g. betweent₂, corresponding to the mid-point 50 of the return stroke and t₃, thelast 10%, 50', of the return stroke), the deceleration must be greaterbut mechanical damping, which begins to take significant effect at aboutt₂, can be effectively utilized. However, this increase in requireddynamic damping pulse energy is partially offset by the increase inmagnetic coupling efficiency due to the increased proximity of thestator 22 and annular disk 18. Therefore, timing is regarded as notparticularly critical to the practice of the invention, but best resultswill be obtained if the dynamic damping pulse is applied after themid-point of the return stroke. It should also be noted in this regard,that as the damping pulse is applied later in the return stroke, thevelocity of the punch 14 will remain high, as shown at 51 for a dampingpulse applied at t₃ (the beginning of the last 10% of the return stroke)of FIG. 4, for a longer period of time. Therefore, the overall punchingcycle time will be reduced and higher punching rates may be achieved.Accordingly, applying the dynamic damping pulse in the last 10% of therebound stroke is considered to be preferred.

Similarly, in the practice of the invention, the accuracy of the energyof the dynamic damping pulse is not particularly critical. Since fairlysubstantial clearance is provided between the rest position of the punchtip 14 and the workpiece 12, it is not necessary to completely avoid thecontact of annular disk 18 and stator coil 22 (or other portion of thepunch mechanism 10) but only that the rebounding motion be less than theclearance shown at d of FIG. 1. Therefore, there is substantialtolerance of actual energy applied both below the optimum energy appliedat the preferred point 50', which would result in underdamping, as shownby punch reference point path 52 and overdamping, shown by referencepoint path 54. However, it should be noted that underdamping ispreferable to overdamping since overdamping increases the duration ofthe return stroke, as noted above in regard to early application of thedynamic damping pulse. Further, the amount of energy absorbed by thepunching action, itself, will vary somewhat from hole to hole and exactcompensation is not to be expected. It is sufficient, therefore, thatthe invention provides substantial working tolerances so that reliablyaccurate punching may be done at rates somewhat exceeding one thousandholes per second (the time reference t₀ +1 msec. is indicated in FIG.4).

Accordingly, the invention may be practiced by providing for a nominaldamping pulse of the same amplitude as the driving pulse and a nominalfraction of the pulse width of the driving pulse with some degree ofadjustment for the amount of punch kinetic energy used in the punchingof the workpiece. As a practical matter, friction of the bushing 16 cangenerally be considered as a constant and, at high punching rates andpunch speeds and accelerations, the action of the spring 24 may beneglected. If such an adjustment for punching energy is provided, theadjustment may be easily made by observing the motion of the punch bodyaudibly or with a transducing arrangement (e.g. a light sourcecooperating with the movable punch body or attached thereto and anoptical motion detector and oscilloscope to provide a time-expandedimage of punch body motion) and adjusting the system to an operatingpoint where contact between annular disk 18 and stator 22 begins tooccur. This point will allow the maximum average operational speed ofthe punching system and provide maximum working tolerance on either sideof the operating point.

Referring now to FIG. 5, the apparatus in accordance with the inventionis shown in schematic form as added to the arrangement of FIG. 1. Astroke control 62 and punch rate control 64, omitted from FIG. 1 in theinterest of clarity are also shown. However, it is to be understood thatother elements of the punch drive system not necessary to anunderstanding of the invention remain omitted from FIG. 5 in theinterest of clarity.

Stroke control 62 is provided for adjustment of the width of the drivepulse 41 in order to impart sufficient kinetic energy to the punch body14' to achieve a cleanly punched hole in the workpiece. Punch ratecontrol 64 determines the interval between drive pulses. Punch ratecontrol is also used to correspondingly adjust the amplitude of thedriving pulse in order to achieve some degree of rebounding from thedownward punch stroke to increase speed of operation. (As noted above,spring 24 is of only marginal assistance in withdrawing the punch fromthe workpiece except at low punching energy.) Of course some mixing ofthese controls could be provided such that both controls affect bothamplitude and width of drive pulse 41. It should be noted that drivecircuit 20, which actually produces drive pulse 41 is essentially formedby two one-shot or trigger circuits and an amplifier. One of the twoone-shot circuits will establish the interval between drive pulses 41and the other one-shot circuit will determine drive pulse duration toterminate the pulse at a predetermined time after the drive pulse isinitiated. The amplifier gain will determine the amplitude of the drivepulse. Voltage control of such circuits including the provision ofnon-linear transfer functions (e.g. to provide a predetermined andpotentially non-linear relationship between punching rate and drivepulse amplitude) is within the purview of those skilled in the art inlight of this disclosure.

To practice the invention in this environment, the same control signalsare provided to a damping drive circuit 20' preferably included, forcircuit simplicity, within drive circuit 20 since the same amplifier canbe used to provide the same pulse amplitude. The damping drive circuitalso essentially comprises two one-shot circuits functioning identicallyto those of drive circuit 20 except that one will be arranged to producea nominal pulse width of about 25% of the pulse width produced by drivecircuit 20. The other one-shot circuit is arranged to control initiationof the damping pulse at a controllable time after either the drivepulse, preferably at or a controllable period after the punch bodyreaches a predetermined position (e.g. the last 10%) of the returnstroke.

For this purpose, optical position detector 66 and an optical system 67is shown for providing a signal to establish timing of the dampingpulse. Pulse position control 68 is provided to provide control of adelay of the pulse subsequent to the signal from the optical positiondetector 66. Provision of such a control may facilitate the physicalplacement of portions of the transducer system, particularly if anoptical system is used. Damping pulse width adjustment 70 is used toprovide a fine adjustment of damping pulse width. However, this controlis neither critical nor necessary to the practice of the invention. Thetransmission of light from a reference point may be done by attachmentof a small light source such as a bulb or light-emitting diode to anyconvenient point of the movable member including the annular disk 18 andthe punch body 14', as generally indicated at 30'. However, otheroptical arrangements are preferred due to the high accelerations, suchas transmission of light through a similarly located aperture orreflection, such as with a mirror or retroreflector. A trace such asthat of FIG. 4 has also been obtained by projecting a light beam betweenstator 22 and annular disk 18 which then acts as a variable aperture,indicating or monitoring punch position by the amount of lighttransmitted through the gap, if any, at a given instant. A similarlyoperating variable aperture could effectively be formed by a reflectoron the lower side of the annular disk 18 or on any portion of themoveable punch member which moves into and out of a light beam over therange of travel of the punch body 14'.

Another transducer arrangement which is presently preferred operates byreflected inductance at radio frequencies and will now be discussed withreference to FIGS. 6-10. An overall schematic circuit diagram of thepunch drive and radio frequency reflected inductance transducing circuitis shown in FIG. 6. Drive circuit 20 and damping drive circuit 20' (FIG.5) are collectively enclosed by dashed line 110. For simplicity, it isassumed in this illustration that the punch drive signals and dampingdrive signals are appropriately generated and applied to a collectivepulse drive circuit 112 at terminal 111. Drive circuit 112 includesvarious amplifier stages for controlling a high voltage, high currenttransistor 120, capable of controlling 100 to 200 volts at currents ofapproximately 1000 Amperes through coil 22 and parasitic capacitance116.

(In practice, a small additional capacitance of several hundredpicofarads is added at 116 to mask variation in collector capacitancewith voltage of transistor 120.) The development of such a high currentfor the brief period of a driving or damping pulse is facilitated by theuse of a large storage capacitor 114.

For monitoring punch position, it has been found that by providing asmall capacitor 122 and a resistor 124 in series with coil 22 (andstorage capacitor 114, which has negligible effect at high frequencies)over connection 118, a series resonant circuit can be formed and drivenat or near a resonant frequency by radio frequency (RF) generator 128with no effect on the punch 14 or its associated drive circuitry 110.Recalling that the coil 22, in combination with annular disk 18functions as a transformer, the inductance of coil 22 will vary slightlywith the coupling and current induced in the annular disk 18, which, inturn, varies with the separation therebetween. Thus, if the frequency ofthe RF generator 128 is set to the resonant frequency of the parallelresonant circuit consisting of capacitor 116 and the capacitance oftransistor 120 and the coil 22 at a particular separation of the annularring 18 from coil 22 (preferably at maximum punch travel), the qualityfactor or Q of the circuit will be changed when the annular disk isdisplaced and the particular reference separation is changed (e.g.annular disk 18 moved closed to coil 22). While the relatively shorttravel of punch 14 is insufficient to accurately monitor punch positionat high resolution with other electrical techniques, the peak-to-peakvoltage change of the RF signal at node 118 will vary by 20% or more,allowing accurate measurement of position.

This variation of peak-to-peak voltage, V_(pp), is plotted as a functionof separation of coil 22 and annular disk 18 in FIG. 7 (e.g. punchposition). To the right of dashed line 130 (generally corresponding tothe rest position of the punch), the variation is relatively linear.Thus, when the punch is operated, the voltage at node 118 will be aradio frequency sine wave having an envelope, over time, determined bythe variation, with time, of the punch position, as illustrated inidealized form in FIG. 8 (except that the radio frequency waveformshould be of much higher frequency than depicted to more fully definethe envelope). In practice, the non-linearity of FIG. 7 causes theenvelope to have a somewhat more pronounced peak which assists inobservation of the desired timing and correction of the non-linearity istherefore not generally desirable. An envelope detector 150 cantherefore provide a voltage waveform which is accurately indicative ofpunch motion over time as the punch is operated. This waveform not onlyallows correct timing of the dynamic damping pulse, described above, butis also useful in providing diagnostics of the punch apparatus.

In a preferred embodiment of the invention, a protective circuit isprovided between node 118 and the envelope detector 150. This protectivecircuit includes several parts including a capacitor for AC coupling andblocking the high DC voltage which will appear on node 118 whentransistor 120 is turned off. Further, to attenuate voltage spikes whichoccur at the instant of turn off of transistor 120, the output side(e.g. node 132') of capacitor 132 is clamped by a pair of oppositelypoled diodes 136. Node 132' is referenced to ground by resistor 134. Asfurther protection, analog switch 138 is also provided to open thecircuit during the duration of the punch drive pulse 41 and, if desired,damping pulse 56, 56' (FIG. 4), as well. The envelope detector circuitis connected to receive an input during intervals of interest after thepunch drive pulse, for example, the period subsequent to the timeindicated by chain line 140 in FIG. 8.

The first stage of envelope detector 150 is preferably formed as avoltage follower amplifier stage 144 which provides current gain. Thisstage can be embodied and is depicted, for convenience, as anoperational amplifier having direct feedback to the inverting input. Ofcourse, other embodiments may be used since the particularimplementation is not critical to the practice of the invention. If,however, an operational amplifier is used, as depicted, resistor 142 isnecessary to provide base current to an input transistor at thenon-inverting input thereof.

The second stage of the envelope detector 150 is preferably a voltagegain stage, preferably embodied with an operational amplifier 156 havingvoltage gain determined by resistors 152 and 154 as is well-understoodin the art. Actual envelope detection is done by the following stageincluding a peak detector circuit formed by diode 162 and shorttime-constant RC circuit formed by resistor 166 and capacitor 168. Thispeak detector, by rectifying the RF signal also eliminates one half ofthe waveform appearing at node 118 (e.g. the upper half of FIG. 8, asindicated by a dashed RF waveform). A further operational amplifier 160is used with diode 162 and diode 164 in the feedback circuit in aso-called ideal diode configuration to compensate for the inherentforward drop of diode 162. Resistors 167 and 171 determine the gain ofthis stage.

By allowing monitoring of punch movement during a very large portion ofthe punching cycle, the envelope detector in accordance with thepreferred embodiment of the invention allows either automatic or manualadjustment of the punch drive circuitry to optimize punching for a widevariety of workpiece materials, thicknesses and hole diameters or anyother parameters which affect the energy required for punching. Inaddition, the ability to monitor punch position as a function of timeprovides diagnostic information concerning punch operation, as will bediscussed with reference to FIGS. 9 and 10.

In both FIGS. 9 and 10, normal punching operation is depicted inwaveform A although these waveforms indicate that increased speed ofoperation is possible without dimpling as depicted in waveform B inwhich rebound is evident but insufficient to bring the punch into asecond contact with the workpiece, as discussed above with reference toFIG. 4. Several malfunctions of the apparatus can also be determinedfrom the envelope shape. For example, waveform C represents a conditionwhere the copper annular disk 18 is broken off from the punch shank 14'and tends to float more closely to the coil 22 after delivering kineticenergy to the punch shank until the punch shank rebounds. A similar butfluctuating pattern also indicates binding of the punch body 14' inguide bushing 16; the fluctuation being caused by shifting of dirt orother foreign matter which cause the binding. The length and position ofsegment G generally indicates sticking of the punch in the workpiece.Low drive power is indicated by waveform D. One or more shorted turns ofcoil 22 may appear similar (e.g. in the time shift of the peak) towaveform D if insufficient punching energy resulted (from a reducednumber of ampere-turns) but may also include a general downward shift ofthe waveform due to the reduced Q of the resonant circuit. An open coil22 would interrupt the resonant circuit and be represented by a nearlyflat response, as would a punch stuck in the down position (but at adifferent near constant envelope voltage).

The punch position monitoring arrangement can also reveal othermechanical damage to the punch apparatus such as bent or distorted punchheads which result in the characteristic pattern shown at E of FIG. 10.Further, empirical data regarding the workpiece can be obtained bymonitoring the energy or punch drive power level necessary to achieve afull stroke through the workpiece, as shown at F of FIG. 10 (where therounded peak indicates that rebounding is not occurring although fullenvelope amplitude is obtained.

It should also be noted that the preferred RF reflected inductanceposition transducer described above is somewhat more sensitive thanother transducing arrangements and, since it directly monitors theposition of the annular ring 18, is capable of producing more diagnosticinformation than other transducer arrangements. For example, opticaltransducing of punch body position would not reveal detachment of theannular ring 18 from the punch body. Further, due to the non-linearitiesof the RF reflected inductance transducer arrangement in comparisonwith, for example, optical or capacitive transducers, diagnosticinformation appearing in waveforms such as waveform C of FIG. 9 andwaveform E of FIG. 10 can be more readily resolved and interpreted. Atthe same time, the response of the RF reflected inductance transducingarrangement is sufficiently linear to be readily understood by anoperator and related to the actual punch movement.

While a capacitive analog of the RF reflected inductance sensing ofpunch position is theoretically possible, as a practical matter it woulddiffer in several significant ways and therefore is not preferred. Mostevidently, the punch structure and all electrical components in thedrive and sensing circuit all have parasitic capacitances to groundwhich are significantly large in comparison to the capacitance betweenthe coil 22 and the annular disk 18. Further, the inductance of coil 22is sufficiently large to prevent resonance at a frequency wheresignificant variation in peak-to-peak voltage could be detected, evenwith the formation of a relatively large capacitor having at least oneplate isolated from ground (which would, itself be difficult due to thedifficulty of forming a reliable connection or electrical isolationbetween punch shaft 14' and bushing 16 and the air dielectric of such acapacitor). Thus, a separate tank circuit would be required including anadditional coil and attachment of additional capacitor structure to thepunch, increasing its mass. Correspondingly, the coil 22 would be takenout of the sensing circuit and malfunctions involving coil 22 could notbe sensed. In contrast, the preferred RF reflected inductance sensingrequires no mechanical or electrical changes to the punch drive circuitand does not affect the operation of the punch in any significant way.

As noted above, neither of controls 68 and 70 are considered necessaryto the successful practice of the invention. However, of these twocontrols, damping pulse width control 70 is more likely to be usefulduring operation of the invention in order to compensate for a range ofenergies absorbed during the punching operations. However, as apractical matter, the punch rate and stroke controls will generally beset with respect to the properties of the workpiece material so that thekinetic energy of the punch body 14' will be about the same during theupward portion of the stroke, regardless of the punching energyrequired. That is, the energy dissipated in the workpiece should besmall in comparison to the energy imparted to the punch by the drivepulse and transferred to the upward portion of the stroke by the elasticrebound of the punch. Damping pulse position control 68 could also beused if the desired point of the upward stroke could not be monitored orobserved due to the geometry of the punching mechanism 10 and or toprovide a fine control over the position detected by optical positionsensor 66. If either of these controls 68, 70 are provided, adjustmentis preferably done by observing motion of the punch body 14' by means ofan oscilloscope 72 (preferably of the dual trace type) displaying thesensed punch position in accordance with any of the above transducerarrangements described above which will produce a display of waveformssimilar to that of FIGS. 4, 9 and 10 and the amplitude of the reboundmotion 58 directly observed during adjustment of the system.

While not considered necessary to the successful practice of theinvention, the dynamic damping could, of course be more rigorouslydetermined and/or more closely controlled. Specifically, the kineticenergy of the punch body 14' during the upward stroke will be determinedby the kinetic energy of the downward stroke, less the energy absorbedduring the punching operation, the friction in bushing 16 and theadiabatic and non-adiabatic air losses in the pneumatic damping providedby the annular disk 18 and stator 22 because all other interactionsinvolved in the rebound of the punch body 14' are substantially elastic.Both friction and air losses can be integrated over a full stroke andthis integrated value is substantially constant for a given punchingrate. Therefore, if the shear and tensile strength of workpieces isrelatively consistent, the damping pulse width would vary inversely withworkpiece thickness at a given punch rate. Similarly, punch bodyvelocity and accelerations vary with punching rate and therefore thevelocity of punch body 14' during the upward stroke varies with punchingrate due to elastic rebounding of the punch body 14'. Therefore, it ispossible to make alterations in the nominal damping pulse width based onthese two parameters from a nominal value corresponding to therelatively constant bushing friction and air losses. Nevertheless, it isconsidered to be both more convenient and more accurate to merely adjustso much of the arrangement of FIG. 5 as is provided based on observationof system performance. This is particularly the case as punching ratesare maximized at rates substantially exceeding those which couldheretofore be obtained with good hole quality. In this latter regard, itshould be appreciated that the dynamic damping pulse width or amplitudecan be increased as far as may be practical with desired increases inpunching rates in order to remove excess kinetic energy from the punchbody 14' during rebound from the punching operation. Therefore, themajor remaining limitation on punching rates lies in the duration ofdamped punch body motion after the dynamic damping pulse is applied.Since the dynamic damping pulse can compensate for some reduction inmechanical damping and since some degree of underdamping is tolerable,two to five fold increases in punching rates are obtainable due to themeritorious effects of the present invention.

In view of the foregoing, it is seen that a simple arrangement has beenprovided which can eliminate dimpling of a workpiece during punchingoperations while substantially increasing the rate at which punchingoperations can be performed. High accuracy and excellent consistency ofhole position and geometry are obtained and maximum punching rates canbe substantially extended in practical applications.

While the invention has been described in terms of a single preferredembodiment, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims. For example, more than a single dynamic damping pulsecould be applied by simple extension of the inventive concepts outlinedabove. In such a case, differing pulse widths and amplitudes of dampingpulses could be employed to essentially halt motion of punch body 14' ator very near the rest position.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent is as follows:
 1. A method for dynamicallydamping a magnetic repulsion mechanism having a primary stator coilmeans and a mechanically damped movable member including a secondarycoil means magnetically coupled therewith across a narrow gap at therest position of said magnetic repulsion mechanism, in which a drivingpulse of a predetermined amplitude and pulse width causes initialmovement of said movable member, said method comprising the stepsofcausing said movable member to rebound from other structure towardsaid primary stator coil means, and limiting rebounding of said moveablemember from said primary stator coil means by applying a damping pulseto said primary stator coil means during said rebound of said movablemember, whereby kinetic energy is removed from said movable memberduring said rebound from said other structure.
 2. A method as recited inclaim 1, further including the steps ofmonitoring an instantaneousposition of said moveable member, and controlling of said damping pulseat or subsequent to a mid-point of said rebound toward said primarystator coil means.
 3. A method as recited in claim 2, wherein said stepof monitoring an instantaneous position of said movable member includesthe steps ofapplying electrical signal to said primary stator coil meansat a resonant frequency of a circuit including said primary stator coilmeans, and detecting an envelope of said electrical signal as thequality factor of said circuit including said primary stator coil meanschanges with position of said secondary coil means.
 4. A method asrecited in claim 3, including the further step ofgenerating atime-expanded image of motion of said movable member in response to asignal representing an envelope detected by said step of detecting anenvelope.
 5. A method as recited in claim 3, including the further stepof generating said damping pulse with a pulse width which is apredetermined fraction of said predetermined pulse width of said drivingpulse.
 6. A method as recited in claim 5, wherein said positionmonitoring step controls application of said damping pulse at orsubsequent to a mid-point of said rebound.
 7. A method as recited inclaim 5, including the further step of generating said damping pulsehaving an amplitude which is approximately the same as saidpredetermined amplitude of said driving pulse.
 8. A method as recited inclaim 3, wherein said position monitoring step includes the stepoftransmitting light from a reference position on said movable member toa light detector.
 9. A method as recited in claim 8, wherein said lighttransmitting step includes transmitting light from an aperture at leastpartially formed by a portion of said movable member.
 10. A method asrecited in claim 5, wherein said step of generating said damping pulseincludes the further step ofadjusting said predetermined fraction ofsaid predetermined pulse width.
 11. A method as recited in claim 10,wherein said position monitoring step includes the further stepofgenerating a time-expanded image of motion of said movable member. 12.A method as recited in claim 1, wherein said movable member furtherincludes a punch tip, including the further step ofpunching an aperturein a workpiece with said punch tip in response to said driving pulse.13. A method as recited in claim 2, wherein said damping pulse isapplied at or subsequent to the last 10% of said rebound toward saidprimary stator coil means.
 14. A dynamically damped magnetic repulsionmechanism includinga movable member which is pneumatically damped by anarrow gap between a primary stator coil means and a moveable secondarycoil means connected to said moveable member, means for applying kineticenergy to said movable member, means for causing rebounding motion ofsaid movable member toward said primary stator coil means, and means forlimiting rebounding of said secondary coil means from said primarystator coil means by removing kinetic energy from said movable memberduring a selected portion of said rebounding motion of said movablemember.
 15. A mechanism as recited in claim 14, further includingmeansfor monitoring position of said movable member including a means fortransmitting light from a reference location on said movable member, anda light detection means.
 16. A mechanism as recited in claim 15, whereinsaid means for transmitting light includes at least one of a light bulband a light-emitting diode.
 17. A mechanism as recited in claim 14,wherein said means for applying kinetic energy includesa pulse generatorfor applying a driving pulse of predetermined amplitude and width tosaid primary stator coil means of said magnetic repulsion mechanism. 18.A mechanism as recited in claim 17, wherein said means for limitingrebounding by removing kinetic energy from said movable member includesafurther pulse generator for applying a damping pulse to said primarystator coil means.
 19. A mechanism as recited in claim 18, wherein saidfurther pulse generator includesmeans for controlling width of saiddamping pulse.
 20. A mechanism as recited in claim 19, wherein saidmeans for controlling width of said damping pulse includesmeans forestablishing said width of said damping pulse as a function of saidpredetermined width of said driving pulse.
 21. A mechanism as recited inclaim 19, wherein said means for controlling width of said damping pulsefurther includesmeans for establishing said width of said damping pulseas a fraction of said predetermined width of said driving pulse.
 22. Amechanism as recited in claim 15, wherein said means for monitoringposition of said moveable member further includesmeans for detecting atleast a predetermined point in a range including at least a mid-pointand a last 10% of said rebounding motion of said moveable member towardsaid primary stator coil means.
 23. A mechanism as recited in claim 14,wherein said movable member further includesa punch tip for forming anaperture in a workpiece in response to said means for applying kineticenergy to said movable member.